1. Field of the Invention
The present invention relates to a display device that is mounted in a portable telephone, an information terminal, a handy terminal, or the like. In particular, the present invention relates to a connection structure of a driver IC that drives two display elements that are arranged in front and in back. Display elements such as STN and TFT liquid crystal elements, organic EL elements, FEDs, and LEDs can be used as the display elements.
2. Description of the Related Art
Conventionally, STN liquid crystal display elements are formed by disposing two transparent substrates, on which transparent electrodes and an orientation film of polyimide or the like are arranged, such that surfaces thereof having the transparent electrodes face each other, and liquid crystals are held in a gap between the transparent electrode surfaces. Segmented electrodes that apply a data signal to a display screen are formed on one of the transparent substrates, and a common electrode that scans lines is formed on the other transparent substrate as a, transparent electrode, thus forming a dot matrix. Applying a time divided voltage to each of the electrodes drives the screen. A segmented signal and a common signal differ from each other. An output electrode of a driver IC is connected to each of the electrodes that constitute the dot matrix, and the signals are applied. The driver IC that outputs the segmented signals and the common signal is constituted by one chip when the number of pixels is on the order of 160×128 dots. For screens having a higher number of pixels, a dedicated driver is used for each segmented signal driver IC and each common driver IC. In some cases, a plurality of driver ICs are used, depending to a relationship between the number of driver signals output from the driver ICs and the number of pixels.
The majority of portable telephones that have been commercialized in recent years are of a shell type that opens into two pieces. A main screen and a back surface sub-screen are mounted to this type of portable telephone as display screens, and display is performed on both screens. The two screens are constituted by separate display panels (refer to JP 2000-338483 A, for example) A method in which screens are displayed by driver ICs that are provided to separately drive each of the display panels, and a method in which two screens are driven by a driver IC that is adapted to the total number of pixels in the display screens of the two display panels, are known (refer to JP 2001-282145 A, for example). For cases where the latter method is used in an STN liquid crystal display device, for example, a film substrate on which the driver IC is mounted is connected to a terminal portion of a main display panel. A connection terminal that supplies an input signal to a sub-display-panel is provided on an edge of the sub-display panel that faces the terminal portion of the main display panel, and the connection terminal and the sub-display panel are connected by a film substrate. Signal electrodes of the main display panel and the sub-display panel share their segmented signals, and display is made on the main panel and the sub-panel by their dedicated common signals. The connection between the film substrate, on which the driver IC is mounted, and the main display panel is made by using an anisotropic conductive film. Common signal electrodes are disposed on both sides of a connection terminal of a segmented signal electrode. The common signal electrodes include a common signal electrode that is used by the main display panel, and in addition, another common signal electrode that is used by the sub-display. panel and that is disposed on an outer side of the common signal electrode of the main display panel. The common signal electrodes are connected by using an anisotropic conductive film. The common signal electrode used by the sub-display panel is wired via an outer circumference of the main display panel. The segmented signal electrode is wired to the connection electrode while being directly drawn out via-the screen.
Further, a display device that performs front and back display by dividing the area of one display panel, and applying the divided portions to the front and the back of a polarizing plate, has been achieved as another method (refer to JP 10-198291 A, for example).
However, problems such as those discussed below exist with the conventional dual screen panels described above.
(1) With the method in which the main panel is connected to the driver IC, and the sub-panel and the main panel are connected by an FPC, (flexible printed circuit), the outer shape of the main panel becomes large because the common electrode that is used exclusively by the sub-panel is formed along the outer shape of the main panel. Further, costs thus increase because the number of main panels that can be cut out from one substrate decreases.
(2) With the method in which the area of the one display panel is divided and applied to the front and the back of the polarizing plate, the placement of the main panel and the sub-panel becomes planar, and this limits design. Further, for cases where panel types such as a transmission color panel and a semi-transmission color panel are combined for the main panel and the sub-panel, color filters become expensive.
(3) Driver ICs for a number of output signals equal to the sum of the number of pixels in the main panel and the sub-panel are necessary in the methods described above. In particular, it is necessary to provide dedicated driver ICs for the common signal output terminals, resulting in increased cost.